1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for distributing a weight on a truck-mounted vibratory source.
2. Discussion of the Background
Seismic data acquisition and processing generate a profile (image) of subterranean geophysical structures. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of the geophysical structures is an ongoing process.
To obtain a high-resolution image of the underground, a seismic survey system employs a seismic source that generates seismic waves, and seismic receivers that record seismic signals associated with the seismic waves. The seismic source imparts energy to the ground. The energy travels through the subsurface and gets reflected from certain subsurface geological formations, e.g., boundaries or layers. The reflected energy travels back to the surface, where the seismic receivers record it. The recorded data is processed to yield information about the location and physical properties of the layers making up the subsurface.
For land explorations, the seismic source may be a vibratory source. A vibratory source may be mounted on a truck and is capable of injecting low-frequency vibrations into the earth by having one part in contact with the earth and oscillating. The energy transmitted by the vibratory source to the ground is proportional with weight acting on it. For land seismic surveys, it is desirable to transmit as much energy as possible to the ground. Thus, the heavier the truck is, the larger the energy transmitted to the ground by the vibratory source.
Geophysical exploration companies install the vibratory sources on buggies for uneven fields or on trucks when road transits are required. Standard vibratory source configurations include power vibratory sources installed on a buggy (or truck). The vibratory sources may be divided into high-, medium- and low-energy sources. High-energy vibratory sources have a weight larger than 10 tons (t). The high-energy vibratory sources are usually installed in the middle of the carrier. Additional weights are used to balance the carrier when using the vibratory source. The additional weights are used to position the center of gravity of the source-vehicle assembly above the vibratory source.
Medium-energy vibratory sources have a weight of around 7 t and the light-energy vibratory sources have a weight of around 3 t. These sources are traditionally mounted at the back of the vehicle. The vehicles carrying medium- and light-energy vibratory sources do not typically need to be balanced with additional weight.
An example of a medium-energy vibratory source and accompanying truck is shown in FIG. 1. This system 10 includes the truck 12 on which the source 14 is provided. The source 14 is attached through a hydraulic cylinder 16 to the truck 12. The hydraulic cylinder 16 is typically attached to the back of the truck, between the frame of the truck and the source. Thus, when the source needs to be taken to a desired location, the hydraulic cylinder lifts the source so it does not touch the ground and the truck can move to the desired location. Once at the desired location, the hydraulic cylinder lowers the source to touch the ground and applies part of the weight of the truck on the source. At this point, the source is ready to efficiently impart energy to the ground.
However, a problem with existing vehicles that carry, either at the back or at the front, a vibratory source is that the percentage of the vehicle weight applied to the source is limited by the design of the vehicle. In addition, an additional weight that is applied on the front wheels (when the source is attached to the back of the truck) when the source contacts the ground and the back wheels are off-ground requires that the front wheels and associated parts be oversized, which is undesirable.
This problem is illustrated with the following example. Assume that for the system shown in FIG. 1, the truck wheelbase length is about 3 m, the distance from the vibratory source to the rear wheels axle is about 1.5 m, the gross weight of the vehicle (including the source) is about 15 t and the distance from the center of gravity of the system, when the source is up, to the front wheels axle is about 1.5 m. Using basic physics, i.e., the sum of total forces on the Y axis should be zero and the sum of the total torque produced by these forces is also zero, the following results are obtained. When the source is up, the weight on the front wheels is about 7.5 t and the weight on the back wheels is about 7.5 t. When the source is lowered to contact the ground so that the back wheels are completely off the ground, the weight on the front wheels is about 10 t, the weight on the back wheels is zero and the weight on the vibratory source is about 5 t.
Thus, it can be seen from this example that for achieving a weight of 5 t on the vibratory source, the front wheels need to be designed to support 10 t. It would be advantageous to find a solution to apply less weight on the front wheels and more weight on the vibratory source. Therefore, there is a need in the industry to provide a simple, reliable and cost-effective system of distributing more weight on the vibratory source and less weight on the wheels.